1. Field of the Invention
The present invention relates generally to a ring gating technique and more specifically to an improved ring gating technique and apparatus which enables the formation of molded plastic products which have a clean through hole and which exhibit even stress and minimal flow marks.
2. Description of the Prior Art
Known mold gate systems are common in design and mode of operation. That is to say, hot plastic material is usually distributed through a hot manifold to a heated nozzle assembly. A valve gate pin within the heated nozzle is either hydraulically, pneumatically or mechanically retracted from a gate orifice in the mold cavity and hot plastic is injected therethrough into the mold cavity. After the cavity is filled, the valve gate pin is returned to its closed position wherein it seats against the gate orifice to cut-off the flow of plastic.
However, when the mold is opened and the plastic part is ejected, inevitably a "gate pin mark" impression can be seen on the surface of the part at the location where the plastic was injected.
The desired position for the injection mold gating of circular or square parts is usually in the center of the part. When a through hole is required to be formed at the center of the part, the gating must be offset from the center and use made of hot tip, cold runner, or gate valve techniques. However, these techniques tend to result in uneven filling, create molded-in part stress and a produce a weld line which originates at the core pin which forms the hole. Viz., as shown in FIGS. 1 and 2 the hot plastic flow (f) which is injected via a heated probe or conventional valve gate 1, enters a mold cavity 2 at a location or gate point (p) and separates (as indicated by the bold arrows) into two flows which pass around on either side of the projection or pin 3 which is used to form the required opening. When the two flow fronts meet on the other side of the pin, the undesirable weld line (w) is created.
Alternatively, instead of using the above mentioned offset injection technique, the part may be centrally gated in the area of the hole which is formed, using hot or cold multi-tipped edge gating in the manner illustrated in FIGS. 3 and 4. However, this technique results in a less than perfect surface at each gate point and also produces flow lines in the molded part originating between the individual gate locations. Viz., as illustrated in FIG. 4, a number of flow or weld lines are created by the multiple flows of plastic which result from injection by way of a sprue 4 having multiple sub-gates. Viz., as shown, in the case wherein the sprue has three sub-gates, three separate flows are produced within the mold cavity. Upon the flow fronts meeting one another, three flow or weld lines are produced.
A more laborious and less desirable technique of producing the above type of perforate part involves molding the part without the hole and then forming the opening using a separate drilling or punching operation. While this generally solves the flow line problem, it introduces the need for a number of additional operations to be performed and thus increases the cost of the article.